Prospective surveillance of hospital-acquired infections was undertaken in the geriatric ward of the University Hospital, Utrecht, the Netherlands. The medical records of 300 patients were studied for the presence of nosocomial infections using the criteria defined by the Centers for Disease Control (CDC), Atlanta, Georgia, USA. Data were collected from patients with and without infection, which allowed for the analysis of risk factors for nosocomial infection. In 100 out of 300 patients (33.3%), a total of 126 infections was diagnosed. The incidence of nosocomial infections was 16.9 per 1000 days of stay in the hospital. The mean length of stay of patients with infection was 39 days, while that of patients without infection was 17.8 days. Infections developed after an average stay of 13.3 days in the hospital. Patients with infections were 2.6 years older than patients without infections (P = 0.005). Dehydration was shown to be a major risk factor for infection (RR = 2.1, 95% CI: 1.4-3.2). Of the infections, 58.7% were urinary tract infections (UTIs, asymptomatic and symptomatic). The most important risk factor for an asymptomatic UTI was an indwelling urinary catheter (RR = 7.3, 95% CI: 3.1-17.1). The duration of use of the indwelling urinary catheter was of significant influence in the development of a UTI. Seventy percent of the patients with an asymptomatic UTI were treated with antibiotics. Infections of the gastrointestinal tract accounted for 19.8% of all nosocomial infections. The majority of these infections were due to an outbreak of Clostridium difficile. In conclusion, the length of stay may be prolonged by a nosocomial infection. In this study, the main risk factors for developing a nosocomial infection were age, dehydration and the presence of an urinary catheter. Our observations showed that age is a predisposing factor for nosocomial infection and that the risk increases with each year, even for geriatric patients.